The present invention relates to a process for the preparation of 1,1,3-trialkoxypropane by the acid-catalyzed reaction of acrolein with a lower alcohol. The process is particularly suitable for the continuous preparation of 1,1,3-trialkoxypropane with higher selectivity than previous methods.
The preparation of 1,1,3-trialkoxypropane by reaction of acrolein with a lower alcohol in the presence of hydrochloric acid or sulfuric acid as catalyst, partial or complete neutralization of the reaction mixture, and recovery of the same by distillation is known (see DE 898 895; R. H. Hall and E. S. Stern in J. Chem. Soc., 3388 to 3393 (1954)). In the acid-catalyzed reaction of acrolein (I) with a lower alcohol, not only is the desired 1,1,3-trialkoxypropane (IV) formed, but 3-alkoxypropionaldehyde (II) and acrolein dialkyl acetal (III) are also produced. As the following reaction scheme shows, equilibria exist between the above-mentioned substances (I) to (IV) in the presence of the acid catalyst: ##STR1## Owing to the complex equilibrium positions between the compounds (I) to (IV) and to the fact that further acrolein derivatives can be formed from acrolein and from the water arising during the formation of acetals, it has hitherto been difficult to obtain the desired 1,1,3-trialkoxypropane with higher selectivity and in higher total yield. In order as far as possible to avoid a reverse splitting of (IV) during the recovery of the reaction mixture by distillation, at least a partial neutralization of the reaction mixture prior to distillation is necessary. A disadvantage of the previously known process, in addition to the often unsatisfactory yield, is that the salts obtained in the reaction mixture from the neutralization of the catalyst lead to encrustations in the distillation equipment--encrustation of this kind is not acceptable in a large-scale plant.
According to DE 898 895 the equilibrium of the reaction can be shifted in favor of the desired trialkoxypropane by carrying out the reaction in the presence of substances which bind water. Such a measure is, however, very expensive on a commercial scale. DE 898 895 also includes instructions to azeotropically dehydrate, by means of benzene, the 3-methoxypropionaldehyde which forms as a by-product in Example 3 of DE 898 895 and is obtained mixed with water during recovery by distillation, and to add it to a subsequent batch. The disadvantage here is that a further organic solvent is necessary for the dehydration.